There is an application note by Agilent, which describes how to add the signals from a waveform generator and a DC source, in order to get offsets up to 42 V (the limit for the 33120), by connecting them in series. In principle, this should work within the bandwidth of the DC source.

I tried this with two 33120 : one generating a sine, the other a DC voltage. The ac amplitude drops beyond about 100 kHz. Is this the limit from the 3310 bandwidth ?

I tried this with two 33120 : one generating a sine, the other a DC voltage. The ac amplitude drops beyond about 100 kHz. Is this the limit from the 3310 bandwidth ?

Each 33120A has 4000 pF to ground and impedance 50 Ohm. Thus, there will be a voltage drop set by the current flowing to ground through 50 Ohm + 4000 pF. This should determine a frequency cutoff in the range of a few MHz. This would not happen if both 33120A had not common grounds.

So, one should :

- float one of the two 33120A ?

- by-pass the DC source for AC, using a filter made of inductance (about 1 mH) and a capacitance (about 1 muF) ?